[libTooling] Add `ifBound`, `elseBranch` RangeSelector combinators.

Summary:
Adds two new combinators and corresponding tests to the RangeSelector library.
* `ifBound` -- conditional evaluation of range-selectors, based on whether a
   given node id is bound in the match.
* `elseBranch` -- selects the source range of the else and its statement.

Reviewers: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67621

llvm-svn: 372410